public class cengxutree {
    class Solution {
        public List<List<Integer>> levelOrder(TreeNode root) {
            List<List<Integer>> ret = new ArrayList<>();
            Queue<TreeNode> queue = new LinkedList<>();
            if(root==null){
                return ret;
            }
            queue.offer(root);
            while(!queue.isEmpty()){
                int size = queue.size();
                List<Integer> list = new ArrayList<>();
                while(size > 0){
                    TreeNode cur = queue.poll();
                    list.add(cur.val);
                    size--;
                    if(cur.left != null){
                        queue.offer(cur.left);
                    }
                    if(cur.right != null){
                        queue.offer(cur.right);
                    }
                }
                ret.add(list);
            }
            return ret;
        }
    }
}
